1. Field of the Invention
The present invention relates to a method and apparatus for net shape precision ausform finishing of rolling element bearing races by controlled induction heating and deformation devices to produce contacting surfaces with enhanced strength and durability by the application of thermal-mechanical techniques.
2. Description of the Prior Art
Ball and roller element bearings are critical machine components used in high performance drive train transmissions, and are heavily loaded with contact stresses of up to 250 K psi while operating over a broad speed range. Such rolling element bearing races require high surface strength for resisting contact fatigue, wear and plastic deformation, as well as high strength and toughness in the core with adequate fracture and crushing resistance. Furthermore, bearing races must be precision finished to high dimensional accuracy and fine surface finish to ensure interchangibility of parts and to minimize vibration and fatigue loading. Such a combination of mechanical properties and dimensional accuracy is achieved utilizing a complex manufacturing process sequence consisting of initial rough machining to approximate size, heat treatment to achieve the desired gradient of mechanical properties, and finally hard grinding and related processing steps for precision finishing to final dimensions. Optimal material properties exist in the as-hardened condition in terms of its surface fatigue response. However, the beneficial as-hardened near surface layers are removed by hard grinding to achieve the desired dimensional accuracy, thereby redressing the prior manufacturing errors and heat treatment distortions. Hard grinding is expensive and can be detrimental if grinding cracks and burns are produced due to abusive practice, requiring etching type inspection techniques, thereby further adding to production cycle time and costs. A method and associated apparatus are disclosed for integral surface heat treatment and precision finishing of rolling element bearing races, thereby eliminating the need for traditional hard grinding and related finishing operations.
The process disclosed by this invention, utilizes contour induction heating to austenitize the surface layers of the bearing races, followed by rapid quenching in marquenching oil maintained at appropriate temperature of up to about 600.degree. F. to achieve a metastable austenitic condition in the surface layers. The surface layers in this metastable austenitic condition are then precision ausform finished to final dimensions and then quenched for transformation to martensite. The bearing race ausform finishing thus integrates the surface induction heating process with a precision roll finishing operations to net shape finish the contacting surfaces of roller element bearing inner and outer races.
Most bearing races are made of high carbon through-hardening type steels such as AISI-52100, whereas bearings used in more heavily loaded and critical transmissions are made of low carbon low-alloyed steels such as AISI-8620 which are case-carburized to produce a hardened case combined with a tough core. The present invention is applicable to both through hardening and carburizing grade bearing steels. Through-hardening steels are traditionally hardened by first austenitizing or heating over the upper critical temperature (approximately 843.degree. C. or 1550.degree. F.), and then rapidly quenching to about the room temperature or below to achieve desired martensitic transformation, followed by a tempering cycle to toughen the core material. The microstructure of such quenched and tempered AISI-52100 comprises plate martensite, alloy carbides and retained austenite; the surface hardness and amount of retained austenite depends upon the tempering temperature used. The heat treatment of carburizing grade surface hardening type steels require additional processes to case-carburize the components prior to the hardening and tempering steps. For through hardening steels, the present invention has the additional advantage of eliminating all batch manufacturing operations such as furnace heat treatment for hardening, and instead in-line induction heating and integral quenching is used.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.